The present invention uncovers a two channel stereo display, which simultaneously generates a right and left image in discrete modulation channels, which differ by the polarization of their light beams. The invention relates to display and projection systems using micro electro mechanical systems (MEMS) as displays. More specifically, the invention relates to the chirality (handedness) of deflectable micro mirror devices (DMD) caused by the orientation of their mirror deflection axes and uncovers solutions for MEMSs which have their mirror deflection axes oriented parallel to a symmetry axis of the image raster of the MEMS.
A polarization coded 2 channel stereo display system requires two spatially separated and different linearly polarized light beams which are simultaneously modulated by at least two SLMs upstream to the superposition system (polarization combiner). When two polarized beams are combined, one light beam (“S” polarized light) is folded into the other (“P” polarized light), the direction of which remains unchanged. This is common to all polarization combiners (PBS, e.g. MacNeille beam splitters; wire grid polarizers WGP, Moxtek Inc, UT, USA). The folding corresponds to an image reflection. As the modulation task of the SLMs takes place upstream of the superposition and only one beam is reflected (mirrored), one of the two SLMs has to create the reflected image. This can easily be achieved for liquid crystal SLMs by mirror symmetrically addressing the modulators of the two channels. Light is incident on these LC modulators with an incidence angle of 0° (perpendicular to the surface of the modulator). Light paths therefore are not influenced by rotation or mirroring.
Obviously MEMSs can also be addressed mirror symmetrically. However MEMSs of the state of the art (e.g. DMDs by Texas Instruments) do not show any axes of symmetry considering their overall operation. Only the “On” beam is reflected perpendicular to the modulator surface. The incident beam however is directed perpendicular to the mirror deflection axis (MDA), which is rotated by 45° to the image raster. It also reaches the display under an incidence angle of twice the deflection angle of a single mirror (FIG. 1). Thus, with a single DMD type as described no symmetrical light paths are possible.
A DMD with a 3×4 matrix is shown in FIG. 2A (this corresponds to the predominant width/height ratio of 4/3). Single mirrors (17) rotate around a deflection axis which is has an angle of 45° relative to the raster image. In FIG. 2B the single mirror deflection axes are visible after “removing” the mirrors. The center DMD corresponds to the state of the art type from Texas Instruments (U.S. Pat. No. 5,600,383). While the raster image shows internal symmetry, this is no longer the case if the orientation of the mirror deflection axis is taken into account. For overall operation no internal symmetry exists. After any mirroring (only horizontal and vertical mirroring is shown) this central “L” topology is converted into a single “R” topology. Due to their rectangular shape and to the orientation of the mirror deflection axes, which are rotated 45° to the image raster, these MEMS show stereo isomery. Stereo isomery is characterized by the existence of two different topologies which are mirror symmetric and cannot be transformed into each other by rotation.
In the prior art, when more than one MEMS is used (e.g. color generation in 3-chip designs) the second stereo-isomeric type is optically mimicked by using an additional reflection surface. This results either in equalizing the number of reflections downstream the MEMSs (e.g. Kavanagh et al., U.S. Pat. No. 5,638,142) or having an even-numbered difference of the number of reflections downstream the MEMSs (e.g. Fielding et al., U.S. Pat. No. 6,250,763). This was enforced by using the same physical layer for splitting and combining the input and ON-beams. In these arrangements, the mirror deflection axes of the single mirrors of all MEMSs are coplanar to the plane of superposition.
In our application U.S. Ser. No. 11/017,916 we have uncoupled split and combine systems (e.g. FIG. 3), and uncovered new solutions all without the additional reflection surface needed in the prior art mentioned above: this can be realized e.g. by coupling stereo-isomeric pairs of MEMSs resulting in mirror symmetric efficient light paths (US 2005 0141076) or by coupling two identical types of a stereo isomeric MEMS at the cost of symmetric light guidance (US 2007 0159680). In this application we focus on a solution where again a single type of MEMS is used without sacrificing the symmetry of light guidance: these MEMSs have their MDAs oriented parallel to an axis of symmetry of their image raster. These MEMSs are internally symmetrical with respect to the mirror deflection axis and image raster.